Aerogels are materials with high porosity and excellent heat insulation properties. Herein, the aerogels refer to solid materials with a porous structure having air as a dispersion medium, and specifically solid materials having a porosity of 60% or more. The porosity represents the amount of air contained in an apparent volume of a material by a volume percentage. Thermal conduction in a material involves solid conduction (transmission of thermal vibrations), convection, and radiation. In general, contribution of convection to thermal conduction is largest in a material having a large porosity. On the other hand, since the aerogels have a very small pore size of about 10 to 100 nm, movement of the air through the voids is greatly obstructed and thermal conduction by convection is significantly inhibited. As such, the aerogels have excellent heat insulation properties.
A method for producing an aerogel is disclosed in which hydrolysis of alkoxysilane as a starting material and polycondensation of a resulting product are carried out to obtain a gelatinous compound; and the gelatinous compound is dried under a supercritical condition of a dispersion medium (Patent Document 1). Another method is also known in which an alkali metal silicate as a raw material is passed through a cation exchange resin or is added a mineral acid, to prepare a sol; the sol is gelated; and thereafter, the gel is dried under a supercritical condition of a solvent (Patent Document 2). The aerogels (silica aerogels) made by such methods have a fine silica framework, and therefore exhibit excellent mechanical strength despite their high porosity.
The known methods described above make it possible to manufacture an aerogel having a high porosity by drying and removing the dispersion medium in the gel under the supercritical condition of the dispersion medium and thereby replacing it with air while inhibiting drying shrinkage of the gel. However, since enormous costs are needed to realize the supercritical conditions, actual applications of the aerogels obtained through drying under the supercritical conditions are limited to those that are worth such high costs. Accordingly, a method of drying at atmospheric pressure for the purpose of cost reduction is suggested (Patent Document 3).